La plaque de terre est sur la table.

Because plaque and table are both feminine singular nouns in French.

• la = the feminine singular definite article
• plaque = feminine
• table = feminine

So:

• la plaque = the plaque / slab
• la table = the table

If the noun were masculine, you would use le instead.

Here, de terre is acting like a noun complement: it tells you what kind of plaque it is, or what it is made of / associated with.

French often uses de + noun with no article in this kind of structure:

• une tasse de thé = a cup of tea
• une table de bois = a wooden table
• une plaque de terre = a slab/patch of earth

By contrast, de la terre usually refers to some soil/earth as a substance or quantity:

• Il y a de la terre sur la table = There is some soil on the table.

So in your sentence, de terre is the normal structure.

Plaque usually suggests something flat, like a plate, slab, sheet, layer, or patch.

So plaque de terre suggests a flat piece or layer of earth/soil rather than just a random lump.

Depending on context, it may or may not sound very natural. If you mean a clod or lump of earth, French might more often use something like motte de terre. If you mean a flat patch/slab of earth, plaque de terre works well.

French usually keeps the article before a noun after a preposition.

So:

• sur la table = on the table
• dans la boîte = in the box
• sous la chaise = under the chair

English sometimes drops articles in places where French does not, but here both languages use the. In general, French is more likely than English to require an article before a noun.

Est is the third-person singular form of être (to be).

The subject is La plaque de terre, which is singular, so French uses the singular verb:

• La plaque de terre est... = The slab of earth is...

If the subject were plural, the verb would change:

• Les plaques de terre sont sur la table.

It can mean several related things, and the exact sense depends on context.

Common meanings of terre include:

• earth as a substance
• soil
• ground
• land
• sometimes Earth as the planet (with context)

In plaque de terre, it most naturally means soil/earth rather than the planet.

Use sur when something is on a surface, usually with the idea of contact.

• sur la table = on the table

Use au-dessus de when something is above something else, not necessarily touching it.

• au-dessus de la table = above the table

So if the earth slab is resting on the table, sur is the right choice.

A careful pronunciation is roughly:

la plak də tɛʁ ɛ syʁ la tabl

A few useful points:

• plaque sounds like plak
• terre ends with a French r sound; the final e is silent
• est is pronounced ɛ; the final -st is silent
• table sounds like tabl; the final e is silent

There is no special liaison in this sentence.

It is grammatically correct, but whether it sounds natural depends on the situation.

• If you really mean a flat slab or patch of earth, it is fine.
• If you mean a bit of dirt/soil on the table, French would more naturally say something like Il y a de la terre sur la table.
• If you mean a clod/lump of earth, une motte de terre may sound more natural.

So the sentence is correct, but the best wording depends on the exact image.

Yes. That changes the nuance.

• La plaque de terre est sur la table.
  This refers to a specific plaque/slab of earth, something already known or identifiable.

• Une plaque de terre est sur la table.
  This introduces it as a plaque/slab of earth, not previously identified.

So the choice between la and une works much like the vs a/an in English.

Yes:

• est: the -st is silent
• terre: the final e is silent
• table: the final e is silent

But in plaque, the final -que is pronounced as k, so plaque is not pronounced like English spelling might suggest. It is simply plak.

Yes. French often uses de + noun to describe the type or material of something.

For example:

• pomme de terre = potato
• battue de terre is not common, but similar structures exist
• pot de terre = earthen pot
• odeur de terre = smell of earth

So plaque de terre follows a very common French pattern: noun + de + noun.